The present invention relates to electronic circuits of the types used in connection with driving an optoelectronic device, for example, a load circuit comprising a laser device, a driver circuit for the laser device, or an electronic device comprising the laser device. The present invention also relates to a control circuit apparatus for dynamically generating a voltage level.
It is known to drive a Vertical Cavity Surface Emitting Laser (VCSEL) using a driver circuit comprising a differential amplifier. The differential amplifier comprises a pair of transistors arranged with a current source to steer current to the VCSEL in response to a differential input signal. The VCSEL has a forward voltage associated therewith.
In contrast to the VCSEL, a forward voltage of a single-mode laser device is lower than the forward voltage of the VCSEL. Consequently, it is not possible to drive the single-mode laser device directly to a lower of two supply voltages (Vee), as is possible with the VCSEL, because the driver circuit lacks sufficient voltage headroom for both the pair of transistors and the current source.
The driver circuit can also comprise a pre-driving stage and an output stage; the output stage draws either no current, or a current equal to a required modulation current for the laser device. However, use of the pre-driving stage results in voltage values corresponding to LOGIC 0 and LOGIC 1 being sensitive to changes in an upper supply voltage. In such circumstances, the voltage values may vary in response to changes in the upper supply voltage.
According to a first aspect of the present invention, there is provided an electronic circuit apparatus comprising: a driving circuit coupled to a single-mode laser device, the driving circuit comprising an output stage, and the output stage comprising an output node for coupling to the single-mode laser device, wherein the single-mode laser device is arranged to maintain the output node at a predetermined potential, thereby ensuring that the single-mode laser is switched by the output stage at an optimum speed.
Preferably, the output stage comprises an active device.
Preferably, the apparatus further comprises means to electrically couple the single-mode laser device to the output stage.
Preferably, there is provided an apparatus, wherein the coupling means is arranged, when in use, to drop between 300 mV and 700 mV. More preferably, the coupling means is a diode.
Preferably, the coupling means comprises a metal-semiconductor junction and may be, for example, a Schottky diode. Alternatively, the diode comprises a p-n junction.
Preferably, the cathode of the single-mode laser device is coupled to the coupling means.
According to a second aspect of the present invention, there is provided a load circuit apparatus for coupling to an output stage of a driver circuit, the apparatus comprising a single-mode laser device and coupling means for maintaining an output node of the output stage at a predetermine potential, thereby ensuring that the single-mode laser is switched at an optimum speed.
According to a third aspect of the present invention, there is provided a driver circuit apparatus for a single-mode laser device, the apparatus comprising an output stage for coupling to the single-mode laser device, wherein the output stage comprises an output node that is maintained at a predetermined potential for ensuring that the single-mode laser is switched by the output stage at an optimum speed.
According to a fourth aspect of the present invention, there is provided the use of a diode for an output stage of a driving circuit for a single-mode laser device, the diode maintaining an output node of the output stage at a predetermined potential, thereby ensuring that the single-mode laser device is switched at an optimum speed.
It is thus possible to provide an electronic circuit apparatus, a load circuit apparatus, a driver circuit apparatus and a use of a diode that permit a single-mode laser device to be driven to the lower supply voltage. Additionally, speeds of falling edges of output current signals are improved. By employing DC coupling, it is thus also possible to avoid losses associated with AC coupling. Also, use of the Schottky diode results in any voltage shift across a range of currents flowing through the Schottky diode being lower than would be obtained if a resistor were used in place of the Schottky diode.
According to a fifth aspect of the present invention, there is provided a control circuit apparatus for dynamically generating a voltage level applied in a driving circuit, characterised by the control circuit comprising an active device operably coupled to voltage level generation means for generating the voltage level from a supply voltage, the voltage generation means being arranged to generate the voltage level in response to a degree of actuation of the active device, wherein the active device is driven in response to the supply voltage.
Preferably, the voltage level generation means is a resistance.
Preferably, the voltage level is inversely related to a reduction of the supply voltage by a voltage drop generated by the voltage level generation means.
Preferably, the active device is an amplifying device. More preferably, the amplifying device is a transistor.
According to a sixth aspect of the present invention, there is provided a driver circuit apparatus for an optoelectronic device comprising the control circuit apparatus.
It is thus also possible to provide a control circuit apparatus that permits an optimum DC bias voltage to be applied to the bases of the pair of transistors of the differential amplifier over a wider range of supply voltages. The above described driver circuit employing a current steering topology can thus be used to drive the single mode laser over a wider range of supply voltages than usually possible using known driver circuits.